TeamCal Pro

TeamCal Pro is a web application supporting project teams in scheduling their members' presence/absence. TeamCal Pro is day-based and that gives it the benefit of easily seeing who's in and who's not and why - and all that in an attractive optical interface.
TCP is powered by PHP and MySql. It supports user authentication, user self-registration, user groups, avatars, absence icons, absence counts, allowances and much more. It comes highly recommended by the users who have suggested many improvements that have been built in.